Production of aromatic fluorides



Ap i 5, 1955 J. D. HEAD PRODUCTION OF AROMATIC FLUORIDES 2 Sheets-Sheet 1' Filed Nov. 25, 1949 IN VEN TOR. James 0. Head Mam A T 7' ORNE KS J. D. HEAD PRODUCTION OF AROMATIC FLUORIDES April 5, 1955 2 Sheets-Sheet 2 Filed Nov. 25, 1949 INVENTOR. James 0 Head BY Mam ATTOR/V YS United States Patent 2,705,730 PRODUCTION OF AROMATIC FLUORIDES James D. Head, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application November 25, 1949, Serial No. 129,263 9 Claims. (Cl. 260-612) This invention relates to an improved method for the production of aromatic fluorides by the thermal decomposition of aromatic diazonium fluoborates.

In 1927 Balz and Schiemann (Berichte, vol. 60, page 1186) disclosed that the thermal decomposition of an aromatic diazonium fluoborate yields the corresponding aromatic fluoride, with the liberation of boron trifluoride and nitrogen. Considerable literature concernit has not been known that the method could be adapted to such commercial scale production. It has now been found that aromatic diazonium fluoborates can be decomposed by a substantially improved method which p gwides a practicable way for producing aromatic fluol'l es.

In the drawings,

Fig. 1 is a schematic flow diagram illustrating the process of the invention.

Fig. 2 is an isometric sketch decomposer shown in Fig. 1.

Fig. 3 is a section of the decomposer taken through the line 3-3 in Fig. 1.

The principal object of the present invention is to provide an improved method for preparing aromatic fluorides from aromatic diazonium fluoborates. Another object of the invention is to provide an improved method for accomplishing the thermal decomposition of the aromatic diazonium fluoborates to the corresponding aromatic fluorides. More specific objects and advantages are apparent from the description and the drawings which illustrate and disclose but do not limit the invention.

According to the invention, an aromatic diazonium fluoborate is applied in a solid state to a surface that is heated to a temperature at which the fluoborate decomof the exterior of the poses to produce the corresponding fluoride in the vaporous state. The aromatic fluoride is then condensed, collected and purified. The method of decomposing a diazonium fluoborate solid state to a heated invention.

The reaction which is believed to occur when an aromatic diazonium fluoborate decomposes is illustrated by Equation 1, below, which shows the production of an aromatic fluoride, nitrogen, and boron trifluoride,

in which R is an aromatic radical. In addition to the simple decomposition reaction indicated above, charring of the organic material almost always accompanies the reaction.

Fig. 1 of the drawings illustrates the overall process of the invention. The diazonium fluoborate (in the form of pellets) to be decomposed is placed in feeding tubes 1, from the bottom of which it is applied to a slowly turning roll 2 (see Fig. 3). Material volatilized on the roll passes through a line 5, and carbonaceous matter is scraped from the roll by a doctor blade 4 (see Fig. 3) and drops into a hopper 6 (see Fig. 3) beneath the roll The roll is driven by a motor 7, power being transm1tted to a wheel 8 through a chain 9. The roll is heated by Dowtherm (the eutectic mixture of biphenyl oxide and biphenyl) which passes from a Dowtherm boiler 10 through a line 11 to the interior of the roll; Dowtherm is returned to the surface is another aspect of the that comprises applying it in a 2,705,730 Patented Apr. 5, 1955 boiler through a line 12 and is there reheated before being The gaseous decomposition products pass they are bubbled through an alkaline solution that fills the tower to about 0 per cent of its height. Substantially all of the desired fluoride is converted to the liquid phase and remains in the tower. Gaseous material passes on to the top of the tower and through a vent at the top thereof into a line Through the line 14, the gaseous material passes into the bottom of a column 15, which is an open column in which entrained liquid is separated from the gases. Non-absorbed gases leave the column 15 through a line 16, and'enter the vacuum side of a steam ejector 17 from which they are discharged from the collection system through a vent 18. The steam ejector maintains the system under vacuum during the decomposition and collection steps of theprocess. Steam to operate the ejector enters through a line 19, a valve 20 being closed during normal operation of the equipment.

An aqueous alkaline absorbing solution is circulated by two pumps 21 and 22; the pump 21 draws the absorbing solution from a receiver 23 and pumps it through a line 24 into the column 13. Absorbing solution flows from the bottom of the column 13 through a line 25, and is returned to the receiver 23 by the pump 22. A determined level of absorbing solution is maintained in the column 13 by means of a section of piping 26 which is raised so that its highest point is the height desired in the column 13. The pressure in the column 15, in the raised section of the pipe 26, and in the receiver 23 is maintained in balance by means of an equalizing tie-in 27 which provides open interconnections among these pieces of equipment.

In the embodiment of the invention illustrated in Fig. l, decomposition is interrupted periodically and the desired aromatic fluoride is separated from the absorbing solution by distillation. The distillation is conducted from the receiver 23 which is equipped with a steam coil 28 which receives steam through a line 29, and which is drained through a line 30 to a steam trap 31. During the progress of this distillation, if desired, auxiliary heat can be supplied to the collector 23 by from the line 19 to the bottom of the collector by opening the valve 20. The aromatic fluorides separated by the distillation are collected in a condenser 32 which is cooled by means of cooling water that is admitted to the lc ondggser through a line 33 and is exhausted through a Figure 2 is an isometric view of one form of decomposer within the scope of the invention. Fig. 2 shows the general exterior appearance of the .decomposer, and illustrates, in particular, the relationship of the line 5 (through which products leave the decomposer) and of the line 11 (through which Dowtherm is admitted to the interior of the roll). A pair of plug cocks 35 is also shown in Fig. 2. The plug cocks are used in conjunction with the pellet feeders, and are equipped with holes that match the holes of the feeders. When the plug cocks are in their normal operating position the holes thereof match the holes of the feeders, and plungers 36 rest on top of the pellets in the feeders and simultaneously drive the pellets against the roll and provide a seal so that air is not admitted through the feeders to break the vacuum inside the decomposer. When it is desired to supply new pellets to the feeders the plungers are removed from the feeders, and the plug cocks are turned to their closed position to prevent loss of vacuum. Additional pellets are then placed in the feeders on top of the cocks; the plungers are replaced; and the plug cocks are turned to their open position so that the new charge is available for feeding onto the roll.

The interior construction of the decomposer is illustrated in Fig. 3. The roll turns (counter-clockwise in the embodiment illustrated in Fig. 3) and acquires a thin coating of the diazonium fluoborate to be decomposed as the feeder area is passed. The diazonium fluoborate decomposes in the course of the revolution of the roll; the aromatic fluoride is removed from the decomposer through the line 5; and carbonaceous residue is removed from the roll by the doctor blade. Alternatively, the diazonium fluoborate, in a comminuted form, is apadmitting steam than about 0.06 0.03 inch) by or a vibrating Certain substituents in the radical R interfere with the diazotization and formation of the diazonium fluoborate. Examples of such interfering substituents include -OH, 03H, NO OH. Accordingly, the improved method of the invention is usually practiced using diazonium fluoborates free of the aforementioned radicals. Halo, alkyl and alkoxy substituents in no way interfere with the diazotization and formation of the diazonium fiuoborate. Furthermore, the use of aromatic diazonium fluoborates having halo, alkyl or alkoxy substituents, and particularly the use of a halo alkoxybenzene diazonium iiuoborate, e. g., 3-chloro-6-methoxy benzene diazonium fluoborate, results in the production of aromatic fluorides having particular utility for the production of oxygen carrying chelates (see J. Am. Chem. Soc., vol. 68, p. 2254 et seq.). Accordingly, a preferred embodiment of the invention involves the decomposition of aromatic diazonium fluoborates wherein the aromatic radical is halo-, alkyl-, or alkoxy-substituted, most desirably one wherein the aromatic radical is both haloand alkoxy-substituted, e. g., 3-chloro-6-methoxy benzene diazonium fluoborate. The halo substituents can be fluoro, chloro, bromo or iodo. The alkyl substituent, or the alkyl part of the alkoxy substituent, can be primary, secondary or tertiary; the number of carbon atoms is limited only by practical considerations. For practical reasons the alkyl or alkoxy substituent usually has not more than about 16 carbon atoms.

Decomposition is effected in the practice of the invention by contacting the diazonium fiuoborate with a heated surface. Neither the nature of the surface nor its shape is critical, as the decomposition can be effected on a curvey or straight surface, composed of a metal, glass, or any material that can be adequately heated (e. g., by a heat transfer medium, by gaseous combustion products, or by radiation) to accomplish decomposition. A rotating cylindrical surface is particularly convenient because carbonaceous matter which forms in the course of the decomposition is removed from the heated surface before new diazonium fluoborate is applied thereto, and this carbonaceous matter can be easily removed from a cylindrical roll, e. g., by a doctor blade as is illustrated in the drawings. It is usually preferred that the heated surface be a metal surface, most desirably nickel, because a metal surface is most conveniently heated. For example, in the embodiment of the invention illustrated in Fig. l a Dowtherm boiler is used to provide heat which is made available by passing Dowtherm through the interior of a metal cylinder.

The temperature at which the diazonium fiuoborate decomposes to produce the corresponding vaporous fluoride is a well known constant for the various known aromatic diazonium fluoborates. For example, Organic Reactions, vol. 5, Roger Adams, Editor in Chief, John Wiley and Sons, Inc., New York, N. Y. (1949), lists the decomposition temperatures of many aromatic diazonium fiuoborates on pages 217 to 226. The boiling points of the aromatic fluorides produced by the decomposition are, of course, also known. Although the decomposition can be carried out at the temperature of decomposition, it is usually preferred that a somewhat higher temperature be used in the process of the invention. This is true because the rate at which decomposition occurs is a direct function of the temperature at which decomposition is carried out. Therefore, using a temperature somewhat higher than the decomposition temperature is usually advantageous because this practice makes possible greater utilization of the equipment. It is usually most desirable to carry out decomposition at a temperature from about 100 C. to about 150 C. higher than the decomposition temperature. When the diazonium fluoborate is 3-chloro- G-rnethoxy benzene diazonium fluoborate, the optimum tem erature is between 290 C. and 310 C.

T e temperature at which the decomposition is carried out and the minimum length of time that must be allowed for decomposition are interrelated. Thus, when the decomposition is carried out in desired procedure (i. e., by applying the diazonium fluoborate to a heated surface I to C. higher than the decomposition temperature of that particular diazonium fluoborate) decompositiori. is effectively accomplished, and the carbonaceous residue is conveniently removed from the about 10 to about 25 seconds are allowed for decomposition to occur. If a temperature lower than the most desired temperature is used to effect the decomposition, a materially longer time must be allowed for the decomposition to be effected (e. g., as much as about 30 minutes if the decomposition temperature approaches the minimum at which decomposition occurs).

When decomposition is accomplished, the products thereof are condensed, alkaline solution. The condensation can be effected in any convenient type of condenser, but it is usually advantageous to carry out the condensation in a unit where the aqueous alkaline solution is contacted directly with the decomposition products. When the condensation is effected in this way the condensation is carried out, and the acidic decomposition products are dissolved in the aqueous alkaline solution simultaneously. An aqueous sodium hydroxide solution is a desirable aqueous alkaline solution, as it is relatively inexpensive and readily available. A solution containing from 1 to 20 per cent of sodium hydroxide is satisfactory for the purpose. (The terms per cent and parts are used herein to refer to per cent and parts by weight, unless otherwise indicated.) it is usually desirable to cool any units (e. g., the tower 13 and the receiver 23) where the decomposition products contact the aqueous solution; the heat of solution of, e. g.,

BFs is thus dissipated. Any convenient method is used to separate the aromatic fluoride from the other materials with which it is associated, but a steam distillation is quite effective for this purpose as other materials present are not sufiiciently volatile that they interfere with such a separation.

The process illustrated in Fig. 1 shows the decomposer and the absorbing systems operated under vacuum. This is not essential to the method of the invention; in fact, it is believed that there would be some advantage to admitting an inert gas to the decomposer to carry the products through the collection system under a slight positive pressure. The inert gas would then be vented from the end of the collection system just as gas that is not absorbed in the absorption system is vented in the process illustrated in Fig. 1. The sole purpose of either vacuum or an inert gas is to carry the products from the decomposer through the absorption system.

The following examples illustrate the new process but are not to be construed as limiting the scope of the mventlon.

Example 1 An aromatic diazonium fiuoborate was decomposed to the corresponding aromatic fluoride according to the following procedure:

3-chloro-6-methoxy benzene diazonium fluoborate was pressed into pellets approximately two inches in diameter by four inches in length under a total pressure of a out 15 tons. These pellets (a total of 27 pounds) were {then melted onto a nickel roll having a length of 10 inches and a diameter of 12 inches, which roll was enclosed in a housing, was rotated at about 3 revolutions per minute, and was maintained at a temperature between about 290 C. and about 300 C. carbonaceous matter which adhered to the roll was scraped therefrom by a doctor blade so situated that the material traveled on the roll about 80 per cent of the circumference thereof before being removed. The diazonium fluoborate was applied to the roll over a period of about 210 minutes. The pressure inside the decomposer and a collection system used to isolate the desired product was maintained between about 5 pounds per square inch absolute and about 6 pounds per square inch absolute. The products of the decomposition of the diazonium fluoborate were removed from the housing surrounding the roll and passed into a tower having an internal diameter of about 4 inches and a height of about 8 feet, which tower was filled with 5 per cent sodium hydroxide to a height of about three feet and was cooled externally to maintain the temperature of the caustic solution between about 20 C. and about accordance with the most at a temperature from 1001C.

heated surface, if from and are treated with an aqueous 25 C. The caustic solution in the tower was removed therefrom at a rate of about one-half gallon per minute, and was pumped into a collector where the organic material settled; the caustic which rose to the top of the collector was returned to the top of the tower at a rate of about one-half gallon per minute. The collector (which contained about gallons of caustic when operation was started) was also cooled externally to maintain the temperature of the caustic solution between about C. and about C. The gases which were not condensed or absorbed in the tower were then passed through a second tower having an internal diameter of about 4 inches and a height of about 5 feet. This second tower was free of absorbing solution, and merely provided a place where entrained liquid could be separated from the gases. The gases from the top of this second column were passed to the vacuum side of a steam ejector to provide vacuum for the entire system. When the organic layer in the collector filled a substantial portion of the volume thereof, decomposition was interrupted, and the -chloro-2-fluoroanisole produced by the decomposition was removed from the collector by a steam distillation. The 4-chloro-2-fluoroanisole so recovered was dried over anhydrous sodium sulfate and purified by fractionation 1n a Podbielniak column; the total recovery of pure compound amounted to 6.2 pounds, which corresponds to a 38 per cent yield of the product.

Example 2 A procedure similar to that described in Example 1 (1) the diazonium fiuobogranular in form, and was was carried out except that: rate was not pelleted, but was fed to the roll by means of roll was operated at a temperature between about 300 C. and about 310 tem was between about 11 pounds per square inch absolute and about 12 pounds per square inch absolute; and (4) 10 pounds of the diazonium fiuoborate were charged in the course of about 90 minutes. The recovery of 165 grams of 4-chloro-2-fluoroanisole corresponded to a conversion of about 6 per cent.

I claim:

1. An improved method of thermally decomposing an aromatic diazonium fiuoborate that comprises (1) continuously conveying the fiuoborate in its solid state into contact with a surface heated to a temperature at which the fiuoborate decomposes to produce the corresponding vaporous fluoride and simultaneously effecting motion of the surface relative to the conveying system to spread the fiuoborate on the surface as a thin coating, (2) col lecting the decomposition products, (3) subjecting the collected decomposition products to the action of an aqueous alkaline solution, and (4) separating the aromatic fluoride from the alkaline solution.

2. A method as claimed matic diazonium fiuoborate has not more than four substituents each of which is of the class consisting of halo, alkyl, and alkoxy.

3. A method of converting 3-chloro-6-methoxy benzene diazonium fiuoborate to 4-chloro-2-fluoro anisole that comprises 1) continuously conveying the fiuoborate in its solid state into contact with a nickel roll heated to a temperature between about 290 C. and about 310 C. and simultaneously rotating the roll to spread the fiuoborate thereon as a thin coating, (2) collecting the decomposition products, (3) subjecting the collected decomposition products to the action of a 5 per cent aqueous a screw conveyor; (2) the C.; (3) the total pressure inside the sys- I in claim 1 in which the aro- 1 all matic fluoride that comprises contlnuously conveymg the poses to the fluoride and simultaneously effecting motion of the surface relative to the conveying system to spread the fiuoborate on the surface as a thin coating.

5. A method according to claim 4 in which the diazonium fiuoborate is applied to the heated surface as a pellet.

6. A method according to claim 4 in which the diazonium fiuoborate is applied to the heated surface in comminuted form.

7. A method according to claim 4 in which the heated surface is a cylindrical metallic surface heated to a temperature from about to about C. higher than the decomposition temperature of the aromatic diazonium fiuoborate.

composes to the fluoride, rotating the roll at a rate such as to spread the fiuoborate on the surface as a thin coating and to allow the fiuoborate thus spread to decompose leaving a carbonaceous residue, and scraping the residue from the surface of the roll before the rotation thereof brings it into the next successive contact with the fluoborate.

9. An improved method of thermally decomposing an aromatic diazonium fiuoborate to the corresponding aromatic fluoride that comprises continuously conveying the fiuoborate in its solid form into contact with a cylindrical metal roll heated to a temperature from 100 to 150 C. higher than the decomposition temperature of the fluoborate, spreading the fiuoborate on the surface of the roll in a thickness not greater than about 0.06 inch while before the rotation thereof brings it into the next success1ve contact with the fiuoborate.

References Cited in the file of this patent UNITED STATES PATENTS 2,115,771 Hellmers May 3, 1938 2,563,796 Shenk et al. Aug. 7, 1951 2,592,809 Kralovec et al. Apr. 15, 1952 OTHER REFERENCES Jour. Amer. Chem. Soc., vol. 69, pages vol. 5, pages 210- N. Y. (1949). 

3. A METHOD OF CONVERTING 3-CHLORO-6-METHOXY BENZENE DIAZONIUM FLUOBORATE TO 4-CHLORO-2-FLUORO ANISOLE THAT COMPRISES (1) CONTINUOUSLY CONVEYING THE FLUOBORATE IN ITS SOLID STATE INTO CONTACT WITH A NICKEL ROLL HEATED TO A TEMPERATURE BETWEEN ABOUT 290*C. AND ABOUT 310*C. AND SIMULTANEOUSLY ROTATING THE ROLL TO SPREAD THE FLUOBORATE THEREON AS A THIN COATING, (2) COLLECTING THE DECOMPOSITION PRODUCTS, (3) SUBJECTING THE COLLECTED DECOMPOSITION PRODUCTS TO THE ACTION OF A 5 PER CENT AQUEOUS CHLORO-2-FLUOROANISLOE FROM THE AQUEOUS SOLUTION. 